path: root/src/lib.rs

//! Parser for the [POM file format](https://www.pom.computer/index.html).
#![cfg_attr(not(feature = "std"), no_std)]
#![deny(missing_docs)]
#![warn(clippy::semicolon_if_nothing_returned)]
#![warn(clippy::redundant_closure_for_method_calls)]

extern crate alloc;
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use alloc::string::String;
use alloc::sync::Arc;
use alloc::vec::Vec;
use alloc::{format, vec};
use core::fmt;
use core::mem::take;

#[cfg(test)]
mod tests;

/// File and line information
#[derive(Clone, Debug)]
pub struct Location {
	file: Arc<str>,
	line: u64,
}

impl Location {
	/// File name
	#[must_use]
	pub fn file(&self) -> &str {
		&self.file
	}

	/// Line number
	#[must_use]
	pub fn line(&self) -> u64 {
		self.line
	}
}

impl fmt::Display for Location {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		write!(f, "{}:{}", self.file, self.line)
	}
}

/// A string value, together with location information about where it is defined.
#[derive(Clone, Debug)]
struct Value {
	value: Box<str>,
	defined_at: Location,
}

/// A parsed POM configuration.
#[derive(Clone, Debug, Default)]
pub struct Configuration {
	/// List of items in configuration, sorted by key.
	items: Vec<(Box<str>, Value)>,
}

impl fmt::Display for Configuration {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		for (key, value) in &self.items {
			writeln!(f, "{key} = {value:?}")?;
		}
		Ok(())
	}
}

/// A parsing or schema error.
#[non_exhaustive]
#[derive(Debug)]
pub enum Error {
	/// I/O error
	///
	/// The first field is a description of what led to the error.
	#[cfg(feature = "std")]
	IO(Box<str>, std::io::Error),
	/// Illegal character in POM file
	///
	/// Specifically, an ASCII control character other than LF, CR immediately followed by LF, and tab.
	IllegalCharacter(Location, char),
	/// Invalid UTF-8 in POM file
	InvalidUtf8(Location),
	/// Couldn't parse signed integer.
	BadInt(Location, Box<str>),
	/// Couldn't parse unsigned integer.
	BadUInt(Location, Box<str>),
	/// Couldn't parse floating-point number.
	BadFloat(Location, Box<str>),
	/// Couldn't parse boolean.
	BadBool(Location, Box<str>),
	/// Opening \[ without matching \].
	UnmatchedLeftBrace(Location),
	/// Key contains invalid characters.
	///
	/// The valid characters are anything outside of ASCII,
	/// as well as `a`–`z`, `A`–`Z`, `0`-`9`, and each of `/.-*_`.
	InvalidKey(Location, Box<str>),
	/// Value contains a null character.
	///
	/// These are not allowed for interoperability with languages
	/// with null-terminated strings (C).
	InvalidValue(Location),
	/// Line is not a `[section-header]` or `key = value`.
	InvalidLine(Location),
	/// Characters appear after a quoted value.
	///
	/// e.g. `key = "value" foo`
	StrayCharsAfterString(Location),
	/// String opened but never closed with a matching character.
	UnterminatedString(Location, char),
	/// Invalid escape sequence appears in a quoted value.
	InvalidEscapeSequence(Location, Box<str>),
	/// Key is defined twice in a file
	DuplicateKey(Box<(Box<str>, Location, Location)>),
	/// Used when there is more than one error in a file.
	///
	/// None of the errors in the array will be [`Error::Multiple`]'s,
	/// and the array will contain at least two elements.
	Multiple(Box<[Error]>),
}

impl fmt::Display for Error {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		match self {
			#[cfg(feature = "std")]
			Self::IO(message, io_err) => write!(f, "{message}: {io_err}"),
			Self::IllegalCharacter(location, c) => {
				write!(f, "{location}: illegal character {c:?}")
			}
			Self::InvalidUtf8(location) => write!(f, "{location}: invalid UTF-8"),
			Self::BadInt(location, value) => {
				write!(f, "{location}: invalid integer: {value:?}")
			}
			Self::BadUInt(location, value) => {
				write!(f, "{location}: invalid (unsigned) integer: {value:?}",)
			}
			Self::BadFloat(location, value) => {
				write!(f, "{location}: invalid number: {value:?}")
			}
			Self::BadBool(location, value) => write!(
				f,
				"{location}: value {value:?} should be off/false/no or on/true/yes",
			),
			Self::UnmatchedLeftBrace(location) => write!(f, "{location}: [ has no matching ]"),
			Self::InvalidKey(location, key) => {
				write!(f, "{location}: invalid key {key:?}")
			}
			Self::InvalidValue(location) => write!(f, "{location}: value contains null characters"),
			Self::InvalidLine(location) => write!(
				f,
				"{location}: line should either start with [ or contain ="
			),
			Self::StrayCharsAfterString(location) => {
				write!(f, "{location}: stray characters after string value")
			}
			Self::UnterminatedString(location, delimiter) => {
				write!(f, "{location}: missing {delimiter} to close string")
			}
			Self::InvalidEscapeSequence(location, sequence) => write!(
				f,
				"{location}: invalid escape sequence {sequence:?} (try using \\\\ instead of \\ maybe?)",
			),
			Self::DuplicateKey(info) => {
				let (key, loc1, loc2) = info.as_ref();
				write!(f, "{loc2}: key {key} was already defined at {loc1}")
			}
			Self::Multiple(errs) => {
				let mut first = true;
				for err in errs {
					if !first {
						writeln!(f)?;
					}
					first = false;
					write!(f, "{err}")?;
				}
				Ok(())
			}
		}
	}
}

impl core::error::Error for Error {
	fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
		#[cfg(feature = "std")]
		if let Error::IO(_, e) = self {
			return Some(e);
		}
		None
	}
}

/// Type alias for [`std::result::Result`] with [`Error`] as the error.
pub type Result<T> = core::result::Result<T, Error>;

fn parse_int(location: &Location, string: &str) -> Result<i64> {
	let bad_int = || Error::BadInt(location.clone(), string.into());
	if !string
		.bytes()
		.all(|c| c.is_ascii_hexdigit() || c == b'x' || c == b'X' || c == b'-' || c == b'+')
	{
		return Err(bad_int());
	}
	let signless = string.strip_prefix(['-', '+']).unwrap_or(string);
	let mut base = 10;
	let baseless = signless
		.strip_prefix("0x")
		.or_else(|| signless.strip_prefix("0X"))
		.unwrap_or_else(|| {
			base = 16;
			signless
		});
	for digit in baseless.bytes() {
		if base == 10 && !digit.is_ascii_digit() {
			return Err(bad_int());
		}
	}
	string.parse().map_err(|_| bad_int())
}

fn parse_uint(location: &Location, string: &str) -> Result<u64> {
	let bad_uint = || Error::BadUInt(location.clone(), string.into());
	if !string
		.bytes()
		.all(|c| c.is_ascii_hexdigit() || c == b'x' || c == b'X' || c == b'+')
	{
		return Err(bad_uint());
	}
	let signless = string.strip_prefix('+').unwrap_or(string);
	let mut base = 10;
	let baseless = signless
		.strip_prefix("0x")
		.or_else(|| signless.strip_prefix("0X"))
		.unwrap_or_else(|| {
			base = 16;
			signless
		});
	for digit in baseless.bytes() {
		if base == 10 && !digit.is_ascii_digit() {
			return Err(bad_uint());
		}
	}
	let val = signless.parse().map_err(|_| bad_uint())?;
	if val > i64::MAX as u64 {
		return Err(bad_uint());
	}
	Ok(val)
}

fn parse_float(location: &Location, string: &str) -> Result<f64> {
	let bad_float = || Error::BadFloat(location.clone(), string.into());
	if !string.bytes().all(|c| {
		c.is_ascii_digit() || c == b'.' || c == b'+' || c == b'-' || c == b'e' || c == b'E'
	}) {
		return Err(bad_float());
	}
	string.parse().map_err(|_| bad_float())
}

fn parse_bool(location: &Location, string: &str) -> Result<bool> {
	match string {
		"yes" | "on" | "true" => Ok(true),
		"no" | "off" | "false" => Ok(false),
		_ => Err(Error::BadBool(location.clone(), string.into())),
	}
}

fn parse_list(string: &str) -> Vec<String> {
	let mut list = vec![];
	let mut item = String::new();
	let mut chars = string.chars();
	while let Some(c) = chars.next() {
		if c == ',' {
			list.push(item);
			item = String::new();
		} else if c == '\\' {
			if let Some(next) = chars.next() {
				if next == ',' || next == '\\' {
					item.push(next);
				} else {
					item.push('\\');
					item.push(next);
				}
			} else {
				item.push('\\');
				break;
			}
		} else {
			item.push(c);
		}
	}
	if !item.is_empty() {
		list.push(item);
	}
	list
}

/// Trait for reading configurations.
///
/// Ordinarily you won't need to implement this trait, since it is
/// already implemented by any `T` implementing [`std::io::BufRead`] (or else `&str` and `&[u8]`,
/// if the `std` feature is not enabled).
pub trait Read {
	/// Read up to the next line feed (or EOF), not including the line feed itself.
	///
	/// Puts the line in `line` and returns `Ok(true)`.
	/// If the end of file has been reached, `line` is unmodified and `Ok(false)` is returned.
	///
	/// You don't need to check for valid UTF-8 here — that is already done in the code which uses
	/// this trait.
	fn read_until_lf(&mut self, line: &mut Vec<u8>) -> Result<bool>;
}

#[cfg(feature = "std")]
impl<R: std::io::BufRead> Read for R {
	fn read_until_lf(&mut self, line: &mut Vec<u8>) -> Result<bool> {
		self.read_until(b'\n', line)
			.map_err(|e| Error::IO("read error".into(), e))?;
		if line.ends_with(b"\n") {
			line.pop();
			Ok(true)
		} else {
			Ok(!line.is_empty())
		}
	}
}

#[cfg(not(feature = "std"))]
impl Read for &str {
	fn read_until_lf(&mut self, line: &mut Vec<u8>) -> Result<bool> {
		match self.split_once('\n') {
			Some((pre, post)) => {
				*self = post;
				line.extend_from_slice(pre.as_bytes());
				Ok(true)
			}
			None => {
				if self.is_empty() {
					return Ok(false);
				}
				line.extend_from_slice(self.as_bytes());
				*self = "";
				Ok(true)
			}
		}
	}
}

#[cfg(not(feature = "std"))]
impl Read for &[u8] {
	fn read_until_lf(&mut self, line: &mut Vec<u8>) -> Result<bool> {
		match self.iter().position(|&c| c == b'\n') {
			Some(i) => {
				line.extend_from_slice(&self[..i]);
				*self = &self[i + 1..];
				Ok(true)
			}
			None => {
				if self.is_empty() {
					return Ok(false);
				}
				line.extend_from_slice(self);
				*self = b"";
				Ok(true)
			}
		}
	}
}

fn is_illegal_byte(c: u8) -> bool {
	(0..0x1f).contains(&c) && c != b'\t'
}

fn process_line(line_buf: &mut Vec<u8>, location: impl Fn() -> Location) -> Result<&str> {
	line_buf.pop_if(|c| *c == b'\r');
	for c in line_buf.iter() {
		if is_illegal_byte(*c) {
			return Err(Error::IllegalCharacter(location(), char::from(*c)));
		}
	}
	str::from_utf8(line_buf).map_err(|_| Error::InvalidUtf8(location()))
}

fn parse_hex_digit(c: char) -> Option<u32> {
	Some(match c {
		'0'..='9' => (c as u32) - ('0' as u32),
		'a'..='f' => (c as u32) - ('a' as u32) + 10,
		'A'..='F' => (c as u32) - ('A' as u32) + 10,
		_ => return None,
	})
}

#[derive(Default)]
struct Parser {
	nonfatal_errors: Vec<Error>,
}

impl Parser {
	fn check_valid_key(&mut self, location: &Location, s: &str) {
		if s.is_empty()
			|| s.starts_with('.')
			|| s.ends_with('.')
			|| s.contains("..")
			|| !s.bytes().all(|c| {
				c >= 0x80
					|| c.is_ascii_alphanumeric()
					|| matches!(c, b'.' | b'_' | b'/' | b'*' | b'-')
			}) {
			self.nonfatal_errors
				.push(Error::InvalidKey(location.clone(), s.into()));
		}
	}

	fn parse_escape_sequence(
		&mut self,
		chars: &mut core::str::Chars,
		value: &mut String,
		location: impl Fn() -> Location,
	) {
		let invalid_escape = |s: String| Error::InvalidEscapeSequence(location(), s.into());
		let Some(c) = chars.next() else {
			self.nonfatal_errors
				.push(invalid_escape("\\(newline)".into()));
			return;
		};
		match c {
			'n' => value.push('\n'),
			'r' => value.push('\r'),
			't' => value.push('\t'),
			'\\' | '\'' | '"' | '`' => value.push(c),
			',' => value.push_str("\\,"),
			'x' => {
				let Some(c1) = chars.next() else {
					self.nonfatal_errors.push(invalid_escape("\\x".into()));
					return;
				};
				let Some(c2) = chars.next() else {
					self.nonfatal_errors
						.push(invalid_escape(format!("\\x{c1}")));
					return;
				};
				let (Some(nibble1), Some(nibble2)) = (parse_hex_digit(c1), parse_hex_digit(c2))
				else {
					self.nonfatal_errors
						.push(invalid_escape(format!("\\x{c1}{c2}")));
					return;
				};
				let char_code = nibble1 << 4 | nibble2;
				if char_code == 0 {
					self.nonfatal_errors.push(Error::InvalidValue(location()));
				}
				if char_code >= 0x80 {
					self.nonfatal_errors
						.push(invalid_escape(format!("\\x{c1}{c2}")));
				}
				value.push(char::try_from(char_code).unwrap());
			}
			'u' => {
				let mut c = chars.next();
				if c != Some('{') {
					self.nonfatal_errors.push(invalid_escape("\\u".into()));
					return;
				}
				let mut code = 0u32;
				for i in 0..7 {
					c = chars.next();
					if i == 6 {
						break;
					}
					let Some(c) = c else {
						break;
					};
					if c == '}' {
						break;
					}
					code <<= 4;
					let Some(digit) = parse_hex_digit(c) else {
						self.nonfatal_errors
							.push(invalid_escape(format!("\\u{{{code:x}{c}")));
						return;
					};
					code |= digit;
				}
				if c != Some('}') {
					self.nonfatal_errors
						.push(invalid_escape("\\u{ has no matching }".into()));
					return;
				}
				if code == 0 {
					self.nonfatal_errors.push(Error::InvalidValue(location()));
				}
				let Ok(c) = char::try_from(code) else {
					self.nonfatal_errors
						.push(invalid_escape(format!("\\u{{{code:x}}}")));
					return;
				};
				value.push(c);
			}
			_ => {
				self.nonfatal_errors.push(invalid_escape(format!("\\{c}")));
			}
		}
	}

	/// Returns (unquoted value, new line number)
	fn read_quoted_value(
		&mut self,
		quoted: &str,
		reader: &mut dyn Read,
		start_location: &Location,
	) -> Result<(String, u64)> {
		let delimiter: char = quoted.chars().next().unwrap();
		let mut unquoted = String::new();
		let mut line_number = start_location.line;
		let location = |line_number: u64| Location {
			file: start_location.file.clone(),
			line: line_number,
		};
		let mut line_buf = vec![];
		let mut first = true;
		loop {
			let line = if first {
				first = false;
				&quoted[1..]
			} else {
				line_buf.truncate(0);
				if !reader.read_until_lf(&mut line_buf)? {
					break;
				}
				line_number += 1;
				process_line(&mut line_buf, || location(line_number))?
			};
			let mut chars = line.chars();
			while let Some(c) = chars.next() {
				if c == delimiter {
					if !chars.all(|c| c == ' ' || c == '\t') {
						self.nonfatal_errors
							.push(Error::StrayCharsAfterString(location(line_number)));
					}
					return Ok((unquoted, line_number));
				} else if c == '\\' {
					self.parse_escape_sequence(&mut chars, &mut unquoted, || location(line_number));
				} else if c == '\0' {
					self.nonfatal_errors
						.push(Error::InvalidValue(location(line_number)));
				} else {
					unquoted.push(c);
				}
			}
			unquoted.push('\n');
		}
		Err(Error::UnterminatedString(start_location.clone(), delimiter))
	}

	fn load(&mut self, filename: &str, reader: &mut dyn Read) -> Result<Configuration> {
		let mut items: Vec<(Box<str>, Value)> = vec![];
		let mut line: Vec<u8> = vec![];
		let mut line_number: u64 = 0;
		let mut current_section = String::new();
		let filename: Arc<str> = filename.into();
		loop {
			line.truncate(0);
			if !reader.read_until_lf(&mut line)? {
				break;
			}
			if line_number == 0 && line.starts_with(b"\xEF\xBB\xBF") {
				line.drain(..3);
			}
			line_number += 1;
			let location = Location {
				file: filename.clone(),
				line: line_number,
			};
			let mut line = process_line(&mut line, || location.clone())?;
			line = line.trim_start_matches(['\t', ' ']);
			if line.is_empty() || line.starts_with('#') {
				// comment/blank line
				continue;
			}
			if line.starts_with('[') {
				// [section.header]
				line = line.trim_end_matches(['\t', ' ']);
				if !line.ends_with(']') {
					return Err(Error::UnmatchedLeftBrace(location));
				}
				current_section = line[1..line.len() - 1].into();
				if !current_section.is_empty() {
					self.check_valid_key(&location, &current_section);
				}
			} else {
				// key = value
				let (mut relative_key, mut value) = line
					.split_once('=')
					.ok_or_else(|| Error::InvalidLine(location.clone()))?;
				relative_key = relative_key.trim_end_matches(['\t', ' ']);
				self.check_valid_key(&location, relative_key);
				let key: String = if current_section.is_empty() {
					relative_key.into()
				} else {
					format!("{current_section}.{relative_key}")
				};
				value = value.trim_start_matches(['\t', ' ']);
				if value.starts_with(['`', '"']) {
					let (value, new_line_number) =
						self.read_quoted_value(value, reader, &location)?;
					items.push((
						key.into(),
						Value {
							value: value.into(),
							defined_at: location,
						},
					));
					line_number = new_line_number;
				} else {
					value = value.trim_end_matches(['\t', ' ']);
					if value.contains('\0') {
						return Err(Error::InvalidValue(location));
					}
					items.push((
						key.into(),
						Value {
							value: value.into(),
							defined_at: location,
						},
					));
				}
			}
		}
		items.sort_by(|(k1, _), (k2, _)| k1.cmp(k2));
		for window in items.windows(2) {
			if window[0].0 == window[1].0 {
				// duplicate key
				self.nonfatal_errors.push(Error::DuplicateKey(Box::new((
					window[0].0.clone(),
					window[0].1.defined_at.clone(),
					window[1].1.defined_at.clone(),
				))));
			}
		}
		match self.nonfatal_errors.len() {
			0 => Ok(Configuration { items }),
			1 => Err(self.nonfatal_errors.pop().unwrap()),
			2.. => Err(Error::Multiple(take(&mut self.nonfatal_errors).into())),
		}
	}
}

impl Configuration {
	/// Load a configuration.
	///
	/// `reader` can be `&str`, `&[u8]`, or anything that implements [`std::io::BufRead`]
	/// (if the `std` feature is enabled) such as `std::io::BufReader<std::fs::File>`.
	///
	/// `filename` is used in error messages.
	pub fn load<R: Read>(filename: &str, mut reader: R) -> Result<Self> {
		// avoid big code size by using dyn reference.
		// the impact on performance is not really important.
		Parser::default().load(filename, &mut reader)
	}

	/// Load a configuration from a file path.
	#[cfg(feature = "std")]
	pub fn load_path<P: AsRef<std::path::Path>>(path: P) -> Result<Self> {
		let p = path.as_ref();
		let filename = p.to_string_lossy();
		let file = std::fs::File::open(p).map_err(|e| Error::IO(filename.clone().into(), e))?;
		Configuration::load(&filename, std::io::BufReader::new(file))
	}

	/// Binary search `self.items`.
	///
	/// See [`std::slice::binary_search_by`].
	fn binary_search_for(&self, key: &str) -> core::result::Result<usize, usize> {
		self.items.binary_search_by(|(k, _)| k.as_ref().cmp(key))
	}

	/// Returns the index of the first key which is greater than `key` + "."
	fn subkey_start_idx(&self, key: &str) -> usize {
		let key_dot = format!("{key}.");
		self.binary_search_for(&key_dot)
			.expect_err("items should not contain a key ending in .")
	}

	/// Returns the index of the first key which is greater than `key` + "." + `x` for all strings `x`.
	fn subkey_end_idx(&self, key: &str) -> usize {
		// NB: / is the next ASCII character after .
		let key_slash = format!("{key}/");
		self.binary_search_for(&key_slash).unwrap_or_else(|x| x)
	}

	/// Extract a section out of a configuration.
	///
	/// More specifically, this will give you the configuration consisting of all
	/// keys starting with `key.` in `self`, together with their values.
	#[must_use]
	pub fn section(&self, key: &str) -> Configuration {
		let start_idx = self.subkey_start_idx(key);
		let end_idx = self.subkey_end_idx(key);
		Configuration {
			items: self.items[start_idx..end_idx].to_owned(),
		}
	}

	/// Get all “direct keys” in this configuration.
	///
	/// More specifically, this returns an iterator of all unique
	/// first components of keys in `self`.
	///
	/// (So if there were keys `sheep.age`, `sheep.colour`, and `farmer-name`,
	///  this would give an iterator yielding
	///  `"farmer-name"` and `"sheep"` in some order.)
	///
	/// The order of items returned is arbitrary and may change
	/// in future versions without notice.
	pub fn keys(&self) -> impl '_ + Iterator<Item = &str> {
		let mut prev = None;
		self.items.iter().filter_map(move |(key, _)| {
			let first_component: &str = key.split_once('.').map_or(key.as_ref(), |(c, _)| c);
			if prev == Some(first_component) {
				return None;
			}
			prev = Some(first_component);
			Some(first_component)
		})
	}

	/// Get all defined keys in this configuration, including nested ones,
	/// and their values.
	///
	/// The order of items returned is arbitrary and may change
	/// in future versions without notice.
	pub fn iter(&self) -> ConfigurationIter<'_> {
		self.into_iter()
	}

	/// Get the list of all “direct children” of `key` in this configuration.
	///
	/// More specifically, this returns an iterator of all unique
	/// (*n*+1)th components of keys in `self` whose first *n* components
	/// are equal to `key` (where `key` has *n* components).
	///
	/// (So if there were keys `sheep.age`, `sheep.colour`, and `sheep.age.unit`,
	///  `subkeys("sheep")` would give an iterator yielding
	///  `"age"` and `"colour"` in some order.)
	///
	/// The order of items returned is arbitrary and may change
	/// in future versions without notice.
	pub fn subkeys(&self, key: &str) -> impl '_ + Iterator<Item = &str> {
		let key_dot = format!("{key}.");
		let start_idx = self.subkey_start_idx(key);
		(start_idx..self.items.len()).map_while(move |i| {
			let this_key = &self.items[i].0;
			let suffix = this_key.strip_prefix(&key_dot)?;
			Some(suffix.split_once('.').map_or(suffix, |(x, _)| x))
		})
	}

	fn get_val(&self, key: &str) -> Option<&Value> {
		let idx = self.binary_search_for(key).ok()?;
		Some(&self.items[idx].1)
	}

	/// Get value associated with `key`, if any.
	#[must_use]
	pub fn get(&self, key: &str) -> Option<&str> {
		Some(self.get_val(key)?.value.as_ref())
	}

	/// Get location in the configuration file where `key` is defined, if any.
	#[must_use]
	pub fn location(&self, key: &str) -> Option<Location> {
		if let Some(val) = self.get_val(key) {
			Some(val.defined_at.clone())
		} else {
			// Check if `key` has any defined subkeys
			let start_idx = self.subkey_start_idx(key);
			let (subkey, subval) = self.items.get(start_idx)?;
			if subkey.starts_with(key) && subkey[key.len()..].starts_with('.') {
				Some(subval.defined_at.clone())
			} else {
				None
			}
		}
	}

	/// Returns `true` if `key` is defined in this configuration.
	#[must_use]
	pub fn has(&self, key: &str) -> bool {
		self.get(key).is_some()
	}

	/// Get value associated with `key`, or else use `default` if it isn't defined.
	#[must_use]
	pub fn get_or_default<'a>(&'a self, key: &str, default: &'a str) -> &'a str {
		self.get(key).unwrap_or(default)
	}

	/// Get value associated with `key`, and parse it as an integer.
	///
	/// Returns `None` if `key` is not defined,
	/// and `Some(Err(…))` if `key` is defined but not an integer.
	#[must_use]
	pub fn get_int(&self, key: &str) -> Option<Result<i64>> {
		let Value { value, defined_at } = self.get_val(key)?;
		Some(parse_int(defined_at, value.as_ref()))
	}

	/// Get value associated with `key`, and parse it as an integer, or else use `default`.
	///
	/// Returns `Err(…)` if `key` is defined but not an integer.
	pub fn get_int_or_default(&self, key: &str, default: i64) -> Result<i64> {
		self.get_int(key).unwrap_or(Ok(default))
	}

	/// Get value associated with `key`, and parse it as an unsigned integer.
	///
	/// Returns `None` if `key` is not defined,
	/// and `Some(Err(…))` if `key` is defined but not an unsigned integer.
	#[must_use]
	pub fn get_uint(&self, key: &str) -> Option<Result<u64>> {
		let Value { value, defined_at } = self.get_val(key)?;
		Some(parse_uint(defined_at, value.as_ref()))
	}

	/// Get value associated with `key`, and parse it as an unsinged integer, or else use `default`.
	///
	/// Returns `Err(…)` if `key` is defined but not an unsigned integer.
	pub fn get_uint_or_default(&self, key: &str, default: u64) -> Result<u64> {
		self.get_uint(key).unwrap_or(Ok(default))
	}

	/// Get value associated with `key`, and parse it as a float.
	///
	/// Returns `None` if `key` is not defined,
	/// and `Some(Err(…))` if `key` is defined but not a float.
	#[must_use]
	pub fn get_float(&self, key: &str) -> Option<Result<f64>> {
		let Value { value, defined_at } = self.get_val(key)?;
		Some(parse_float(defined_at, value.as_ref()))
	}

	/// Get value associated with `key`, and parse it as a float, or else use `default`.
	///
	/// Returns `Err(…)` if `key` is defined but not a float.
	pub fn get_float_or_default(&self, key: &str, default: f64) -> Result<f64> {
		self.get_float(key).unwrap_or(Ok(default))
	}

	/// Get value associated with `key`, and parse it as a boolean.
	///
	/// Returns `None` if `key` is not defined,
	/// and `Some(Err(…))` if `key` is defined but not equal to one of
	/// `off`, `no`, `false`, `on`, `yes`, `true`.
	#[must_use]
	pub fn get_bool(&self, key: &str) -> Option<Result<bool>> {
		let Value { value, defined_at } = self.get_val(key)?;
		Some(parse_bool(defined_at, value.as_ref()))
	}

	/// Get value associated with `key`, and parse it as a boolean, or else use `default`.
	///
	/// Returns `Err(…)` if `key` is defined but not equal to one of
	/// `off`, `no`, `false`, `on`, `yes`, `true`.
	pub fn get_bool_or_default(&self, key: &str, default: bool) -> Result<bool> {
		self.get_bool(key).unwrap_or(Ok(default))
	}

	/// Get value associated with `key`, and parse it as a comma-separated list.
	///
	/// Commas in list entries can be escaped with `\,`.
	#[must_use]
	pub fn get_list(&self, key: &str) -> Option<Vec<String>> {
		let value = &self.get_val(key)?.value;
		Some(parse_list(value.as_ref()))
	}

	/// Get value associated with `key`, and parse it as a comma-separated list, or else use `default`.
	///
	/// Commas in list entries can be escaped with `\,`.
	///
	/// `default` can be anything that can be converted into an iterator of strings,
	/// e.g. `Vec<&str>`, `Vec<String>`, `&[&str]`, etc.
	pub fn get_list_or_default<L>(&self, key: &str, default: L) -> Vec<String>
	where
		L: IntoIterator,
		L::Item: AsRef<str>,
	{
		self.get_list(key)
			.unwrap_or_else(|| default.into_iter().map(|s| s.as_ref().to_owned()).collect())
	}

	/// Merge `conf` into `self`, preferring values in `conf`.
	pub fn merge(&mut self, conf: &Configuration) {
		let mut must_sort = false;
		for (key, value) in &conf.items {
			if let Ok(i) = self.binary_search_for(key) {
				self.items[i].1 = value.clone();
			} else {
				self.items.push((key.clone(), value.clone()));
				must_sort = true;
			}
		}
		if must_sort {
			self.items.sort_by(|(k1, _), (k2, _)| k1.cmp(k2));
		}
	}

	/// Check that `self` follows the given schema.
	///
	/// See the [POM specification](https://www.pom.computer/spec.html) for a description
	/// of schemas.
	pub fn check_against_schema(&self, schema: &Configuration) -> Result<()> {
		_ = schema;
		todo!()
	}
}

/// Opaque type returned by [`<&Configuration>::into_iter`].
#[derive(Clone, Debug)]
pub struct ConfigurationIter<'a>(core::slice::Iter<'a, (Box<str>, Value)>);

impl<'a> Iterator for ConfigurationIter<'a> {
	type Item = (&'a str, &'a str);
	fn next(&mut self) -> Option<Self::Item> {
		let (key, val) = self.0.next()?;
		Some((key, val.value.as_ref()))
	}
}

impl<'a> IntoIterator for &'a Configuration {
	type IntoIter = ConfigurationIter<'a>;
	type Item = (&'a str, &'a str);
	/// See [`Configuration::iter`].
	fn into_iter(self) -> Self::IntoIter {
		ConfigurationIter(self.items.iter())
	}
}